The field of the invention relates to tensioning devices, particularly as used in conjunction with filtration apparatus.
It has been found that the performance of reverse-air-flow baghouses can be effected when bag tension is either insufficient or excessive. Insufficient tension may result in abrasion, flex failures, and/or potential plugging of the fabric. Excessive tension may cause failures in fixed or anchored areas of the fabric. These problems are more fully explained in an article entitled "Make Fiberglass Filter Bags Last Longer By Maintaining Proper Tension" in the March, 1980 issue of Power. Problems are also created by the varying loads applied to the bag during the cleaning cycle and the varying rates of thermal expansion differences between the support structure and the bags.
Current baghouse systems employ a spring tension device where force is a variable due to the linear nature of a tension or compression spring. These tensioning units are externally mounted to the bag cap with the opposite end attached to the baghouse structure by suitable means. Small changes in bag dimension from cleaning or thermal expansion or other causes may result in rapid change in the spring rate with concurrent increases or decreases in bag tension and fiber stress in the bag. Some bags are made of polymeric or similar materials and are subject to the phenomenon known as creep. Creep varies in each polymeric material as a function of temperature and stress. An increase in temperature or fiber stress can cause an increase in bag length with a corresponding decrease in bag tension. Conversely where polymeric materials have had oriented stress imposed upon them during manufacture, shrinkage of the bag could occur, again changing fiber stress on the bag.
It is well known that cyclical application of stress causes fatigue failure. This fatigue failure is accelerated at increased stress levels. Textile materials may also undergo changes in dimension due to fiber straightening or other changes in geometry as a functioning of the load.
Maintaining accurate tension on filter or fume bags is essential to long life and optimum performance. In conventional bag house systems as described above, proper tension may not always be applied. For example, if a conventional spring system has a spring rate of 40 pounds per inch and one desires to set an 80 pound tension, then the spring must be compressed (with a compression spring) or elongated (with a tension spring) two inches to achieve an 80 pound force. A one-quarter inch error in setting the spring will result in a bag tension which is 10 pounds from the optimum. A further and more serious complication exists when a 40 foot high bag house must operate at 500.degree. F. (260.degree. C.). A steel baghouse of this height will expand about 1.7 inches in the transition from 70.degree. F. to 500.degree. F. This expansion would increase the bag tension by 68 pounds, for a total of 148 pounds rather than the 80 pounds desired. Further complications exist since the carbon steel spring will stress relieve about 250.degree. F.